1. Field of the Invention
The present invention relates to charged particle detection and inspection systems, and the invention in particular relates to such systems using multiple beamlets of charged particles.
2. Brief Description of Related Art
A conventional multi-beamlet inspection system is known from WO 2005/024881. The multi-beamlet inspection system disclosed therein is used for inspecting an object, such as a semiconductor wafer. A plurality of primary electron beamlets is focussed in parallel to each other to form an array of primary electron beam spots on the object. Secondary electrons generated from the primary electrons and emanating from respective primary electron beam spots are received by a charged particle imaging optics to form a corresponding array of secondary electron beamlets which are supplied to an electron detection system having an array of detection elements such that each secondary electron beamlet is incident on a separate detection element. Detection signals generated from the detection elements are indicative of properties of the object at those locations where the primary electron beam spots are formed.
By scanning the array of primary electron beam spots across the surface of the object, it is possible to obtain an electron microscopic image of the object. It is desirable to obtain images of a great number of objects per time such that a high throughput can be achieved. For this purpose it is desirable to obtain electron optical images of the inspected surface having a high contrast.
Conventional electron inspection systems using a single primary electron beam, such as scanning electron microscopes (SEMS) use energy filters for increasing image contrast. The energy filter allows secondary electrons exceeding a threshold energy to traverse the filter and to be incident on a detector, whereas secondary electrons having a kinetic energy below the threshold energy are rejected and not allowed to be incident on the detector. Such conventional energy filters used in scanning electron microscopes may comprise a grid electrode disposed in the secondary electron beam path between the surface of the object and an objective lens receiving the electron beam.
It is desirable to have the feature of energy filtering also available in charged particle systems using an array of multiple charged particle beamlets.